Continuous ink jet printing involves the formation of electrically charged drops from a jet of ink, and the subsequent deflection of the charged drops by an electric field to produce an image on a print medium.
In a typical embodiment of a single-jet CIJ printer, electrically conducting ink is forced through a nozzle by applying pressure to the ink. The velocity of the resulting jet of ink must be controlled. This is commonly effected by controlling the constituency of the ink in conjunction with controlling the pressure. Pressure control is usually achieved by varying the speed of the pump producing the flow in response to feedback from a pressure transducer, but it may also be achieved using feedback from a velocity measurement device.
A controlled sequence of drops, each with identical drop volumes and with constant separation between adjacent drops, can then be formed by modulating the jet to give active and controlled drive to the natural process of jet break-up. Jet break-up is usually achieved by carefully modulating the ink pressure, in a sinusoidal manner, at fixed frequency and amplitude; or by modulating the ink velocity relative to the nozzle. A range of options and techniques to introduce pressure modulation, velocity modulation or a combination of both so that uniform drop sequences are obtained are well known in the art.
Charge is induced on individual drops through capacitive coupling. Desired levels of charge are induced on drops by applying a voltage to the charge electrodes at the time the drop separates from the jet. After charging, the drops travel through a constant electric field, formed by applying a high potential difference between two surfaces, whose field lines are perpendicular to trajectory of the jet. Charged drops are deflected by an amount that approximately scales with the charge on the drops.
Un-charged or non-printing drops are collected by a gutter incorporated in the print head, and returned to an ink reservoir in the printer for ink re-flow and re-use.
A significant factor in the reliable operation of a continuous inkjet printer is ensuring that the gutter is capable of collecting all of the non-printing ink drops and that the collected ink is transported back to the ink reservoir.
Typically a continuous ink jet has a relatively small print head that is attached to the printer's ink supply system, reservoir and control electronics via a conduit that is several meters long. The removal of ink collected in the gutter is achieved by drawing the ink, along with air, through a return pipe located in the conduit. This is effected using a vacuum pump.
A number of pumping technologies may used to remove the ink from the gutter. Jet pumps are ideally suited to clearing gas and air mixtures, and produce a uniform and predictable vacuum that efficiently pulls air and ink through the gutter. Jet pump action relies upon turbulence in a pump chamber between a nozzle and a throat in order to work effectively. This pump action has the effect of mixing air and ink efficiently as a by-product of the turbulence that is created to provide the vacuum. Thus, although a jet pump is able to remove ink from the print head effectively, an undesirable consequence is that a large amount of air is trapped in the ink. The air-laden ink will attempt to release air when the ink is returned to the ink reservoir, as the ink is stored at atmospheric pressure rather than at the higher pressure experienced in the return pipe.
The release of air from the air-laden ink in the reservoir often results in the formation of foam, which can cause the ink to overflow from the ink reservoir container. Another problem associated with air-laden ink is that the air can be released from the ink when it is re-circulated through the print head. Such release of air bubbles within the ink system can result in undesirable pressure fluctuations that compromise the subsequent production of droplets.
There are many examples in the art of attempts to reduce the tendency of returned ink to form foams. The extent to which a stable foam forms depends on many factors such as the surface tension of the ink, the partial pressure of solvent vapour present in the reservoir and the rate at which air is released from the ink. Some manufacturers add anti-foaming agents such as surfactants to the ink to reduce foam formation, but this is not preferred as it may affect the adhesion and surface wetting properties of the ink. One example of a physical arrangement to reduce the formation of foam in ink is described in U.S. Pat. No. 6,234,621. This patent describes the returned ink being feed back into the top of an ink reservoir and onto a downwardly spiralling ramp located with within the reservoir. As the ink flows down the ramp its velocity is reduced, the ink spreads out across the ramp surfaces and entrained air is said to be expelled from the ink before the ink added to the upper surface of the ink in the reservoir. This arrangement is expensive to implement and the ramp occupies a significant space within the reservoir.
One alternative arrangement for reducing entrained air in the ink is described in JP9029998 in which the returning air with entrained air is directed onto the upper surface of a sub-chamber. Ink to be returned to the print head is then withdrawn from the base of the sub-chamber. Whilst this arrangement will reduce the amount of air entrained within the ink supplied to the print head, nothing is done to reduce the velocity of the returning ink and thus foam formation and air entrapment is high.
It is an object of this invention to provide a continuous inkjet printer, and/or one or more methods and components therefor, which will go at least some way in addressing aforementioned problems; or which will at least provide a novel and useful choice.